The broad long-term objectives of this work are to develop and apply theoretical models to analyze and to design complementary interactions formed during protein folding and binding. The ability of molecules to recognize one another with appropriate affinity and specificity is central to biology and medicine. The clinical activity of pharmaceutical agents is due largely to their ability to recognize and interfere with one or a small number of molecular targets; undesirable side effects are frequently caused by lack of specificity for the intended target. An important area of research involves understanding the design principles of natural protein molecules and developing tools to engineer modified or entirely new molecules by similar principles. The current proposal focuses on (1) further developments in methodology for the study and engineering of molecular structures and binding partners and (2) applications to particular biological molecules of interest. Methodological enhancements pursued will include improving the robustness of design approaches through a reduction in the rate of false positives, improvement in the balance of packing and electrostatic interactions, and more efficient techniques for treating conformational relaxation. The new methods will be applied to the design and study of novel reagents for structural and cell biology and to computational antibody maturation.